The present invention relates to output of industrial process transmitters. More particularly, the present invention relates to transmitters that communicate using analog signals.
Process instruments are used to monitor process variables, such as pressure, temperature, flow and level, of process fluids used in industrial processes. For example, process transmitters are typically employed in industrial manufacturing facilities at multiple locations to monitor a variety of process variables along various production lines. Process transmitters include sensors that produce an electrical signal in response to physical changes in the process. For example, pressure transmitters include pressure transducers that produce an electrical signal as a function of the pressure of a process fluid, such as in water lines, chemical tanks or the like. Process transmitters also typically include transmitter electronics for receiving and processing the sensor signal so that the transmitter and process can be monitored remotely. Remotely monitored transmitters include electronics that transmit an electrical output over a control loop or network to a central monitoring location such as a control room. Configured as such, the process can be regulated from the control room by including automated switches, valves, pumps and other similar components in the process control system and the control loop.
Advancements in transmitter technology have rapidly increased the amount of information that can be produced by a transmitter. In particular, transmitters can be equipped with multiple sensors to measure multiple process variable inputs or a wider range of a single process variable. For example, transmitters can be provided with multiple pressure sensors as described in U.S. Pat. No. 5,495,769 to Broden et al., U.S. Pat. No. 6,047,244 to Rud, Jr. and U.S. Pat. No. 7,467,555 to Schulte et al., all of which are assigned to Rosemount Inc., Eden Prairie, Minn. Similarly, communications technology has advanced to allow more information and data to be communicated between the transmitter and the control room. For example, digital network protocols, such as HART or FOUNDATION ™Fieldbus, allow data from multiple sensors and multiple transmitters to be transmitted over a common digital communication bus.
Many existing control systems are configured to operate using analog control loops. Many users like the advantages of digital communications, but cannot justify the cost of upgrading equipment and training employees. Typical analog control loops transmit an output representing the measured process variable in the form of a 4 mA to 20 mA signal between the transmitter and control room. For transmitters having a wide ranging sensor or dual range sensors, such as described in the previously mentioned patent to Rud, Jr., the entire measurement range of the sensor or sensors is distributed proportionally along the analog signal. This reduces the resolution of transmitter output along the whole signal, making less information available in particular areas of interest within the sensor range. There is, therefore, a need to increase the compatibility of advanced transmitters with analog control loops. In particular, there is a need for obtaining more information from transmitters operating on analog control loops.